Such an electric arc welding device, such a welding method and such a welding element are known as the present applicant's WELDFAST welding system. In the field of electric arc welding, also known as short-time arc welding or bolt welding, it is known that bolts of various shapes may be welded onto counterpieces in the form of areal metal sheets. Here, the bolts serve as fastening elements, as a rule for plastic clips, which perform a certain function on the counterpiece. As an example, mention may be made of the manufacture of motor vehicle bodies, onto which nowadays many such welding bolts are welded, for the purpose of afterwards fastening lines thereto by means of the plastic clips, fixing internal trim to the body sheet-metal, etc.
Further, it is known that metal nuts may be welded onto metal sheets. Here, the sheets often comprise a hole beforehand, oriented concentrically with an internal thread of the nut, so that objects may be fixed to the sheet by means of screws. When welding nuts onto body sheet-metal, as a rule use is made of a magnetic field so controlled that an electric arc set up between the nut and the counterpiece is moved circularly along the face of the nut. Here, the welding device comprises a coil around the location of the welds, effecting a rotation of the arc on the circular annulus surface of the nut. This is described for example in Bolzenschweissen, Grundlagen und Anwendung by Trillmich Welz, Fachbuchreihe Schweisstechnik, Vol. 133, Deutscher Verlag für Schweisstechnik (DVS-Verlag) GmbH, 1997, Ch. 9.4. A corresponding device is also disclosed, for example, in DE 4,400,957 C1.
For the joining of sheet-metal fastening elements to large-area sheet metal parts, for example bodywork sheets, it is known that a welding segment may be bent off on the fastening element in question, to be placed parallel on the bodywork sheet. The connection is then made by spot welding. A disadvantage here is that a relatively large mass of the sheet metal of the fastening element is required for the welding segment to be laid parallel on the body sheet-metal. With the use of numerous such fastening elements in a motor vehicle, the weight of the vehicle is increased sharply.
To solve this problem, the applicant for the present invention has proposed the so-called WELDFAST welding process. By the WELDFAST welding process, fastening elements in the form of small plates are butt-welded onto a counterplate. This eliminates the parallel fastening segment required in spot welding technology, so that a considerable saving of weight is achieved.
The WELDFAST welding system is often employed for the purpose of butt welding small welding elements in the form of sheet metal parts onto a real counterpieces. A typical example of an application is the production of motor vehicle bodies, where such welding elements are welded onto body parts, and later perform a certain function, for example for laying fuel lines, as anchors for trim, etc. Here, the welding elements comprise a welding segment having an elongated face. Their contour is generally adapted to the contour of the counterpiece.
To facilitate ignition of the electric arc, the contour of the face is often configured tapering at an obtuse angle, departing from the contour of the counterpiece. The electric arc is then set on the counterpiece with the obtuse tip by the reciprocating method of ignition. Then a bias current is applied in order to draw an electric arc from the counterpiece when the welding element is lifted. Then, the welding current is switched on, typically in the neighborhood of up to two thousand amperes, in order to produce an electric arc extending over the entire elongated face of the welding element. Then the welding element is lowered onto the counterpiece, causing the melt formed by the arc to mingle. The welding current is switched off and the welded seam cools out.
The energy required to produce such a welded connection is substantial. Printed source DE 299 05 259 U1 discloses the equipping of a welding device for the WELDFAST welding system with field formers to produce a magnetic field acting upon the electric arc, the latter consisting partly of a soft-magnetic steel. The field formers, according to an example, are provided at the end of the sheet metal, and each encircles it in U-shaped fashion. According to another example, field formers are provided that are arranged parallel with the sheet metal. In both cases, the field formers are to set up a magnetic field acting on the electric arc, the field former thus influencing the spread of the arc. How the electric arc is influenced and how the magnetic field is oriented, however, is not to be learned from this source.